A multiple-input multiple-output (MIMO) wireless communication system includes a plurality of antenna elements at the transmitter and a plurality of antenna elements at the receiver. A respective antenna array is formed at the transmitter and at the receiver based upon the antenna elements associated therewith. The antenna elements are used in a multi-path rich environment such that due to the presence of various scattering objects in the environment, each signal experiences multipath propagation.
MIMO communication systems are advantageous in that they enable the capacity of the wireless link between the transmitter and receiver to be improved. The multipath rich environment enables multiple channels to be generated therebetween. Data for a single user can then be transmitted over the air in parallel over those channels, simultaneously and using the same bandwidth.
Orthogonal frequency division modulation (OFDM) is also effective in multipath environments without involving complicated receiver designs. A combination of OFDM and MIMO techniques has been adapted into various standards, such as 802.11n and Evolved UTRA (E-UTRA), and is promising for next generation wireless data communications.
In an OFDM-MIMO transmitter 10, as shown in FIG. 1, a serial bit information stream 12 is coded by a channel encoder 14 to improve link reliability. The coded serial bit information stream 16 is then punctured by a puncturer 18 to achieve a desirable data rate. The punctured coded serial bit information stream 20 is then interleaved by an interleaver 22 to avoid burst errors.
The interleaved bits 24 are then divided by a multiplexer 26 into multiple serial bit information substreams 28, 30 to increase total data throughput. A plurality of transmitter chains 40 is coupled to the multiplexer 26, with each transmitter chain receiving a respective serial bit information substream 28, 30.
Each transmitter chain 40 includes a serial-to-parallel converter 42 for converting the respective serial bit information substream to a parallel bit information bit substream. In the illustrated example, 3 channel bits 50(1)-50(3), 51(1)-51(3) are provided from the serial-to-parallel 42 to a signal mapper 44. The signal mapper 44 maps the 3 channel bits 50(1)-50(3), 51(1)-51(3) to a channel symbol 52, 53.
A block of the channel symbol 52 is then modulated by an OFDM modulator, such as an inverse fast Fourier transform (IFFT) module 46. The length of the channel symbol block to be modulated by the OFDM modulator 46 is determined by total number of subcarriers. The OFDM modulator converts a frequency domain signal to a time domain signal. The time domain signal is transmitted by a transmit antenna 48(1), 48(2). Effectively, each channel symbol 52, 53 is transmitted on a subcarrier and the channel symbol block occupies the whole bandwidth.
There are several potential problems associated with the conventional OFDM-MIMO transmitter 10. To achieve high data rates in evolving wireless standards, high-order modulation schemes such as 16 QAM and 64 QAM are used. However, these high-order modulation schemes require a higher signal-to-noise (SNR) ratio to achieve certain bit error rates (BER). Modulation schemes that require high SNR to achieve certain bit error rates are adversely affected in multipath fading environments, thus causing the wireless links to be unreliable. Moreover, the peak-to-average ratio (PAR) is high in any OFDM system. High PAR causes problems in RF circuitry design, especially in the power amplifiers.